1. Newton’s Laws of Motion
1st, 2nd, 3rd Law
mass vs weight
coefficient of friction

2. Newton’s 1st Law Law of inertia
“An object at rest stays at rest, and an object in motion stays in motion at the same speed in the same direction, unless acted upon by an unbalanced outside force.”

3. What is Inertia?
Inertia – tendency of an object to continue doing the same thing
When your car stops suddenly, you keep going until the seatbelt or windshield stops you.

4. Law of inertia

5. What is Inertia?
An 18-wheeler, stopped at a light is harder to get going than a little VW Bug. The truck has more inertia because it has more mass.
An 18-wheeler, driving 60 mph is harder to get stopped than a little VW Bug. The truck has more inertia because it has more mass.

6. Newton’s 1st Law

7. Newton’s 1st Law What will happen to the probe as it continues through space?

8. Newton’s 2nd Law Law of Acceleration
Net force is directly proportional to mass and acceleration.
In equation form
Fnet = m a

9. Newton’s 2nd Law Acceleration means “change in motion”
all accelerations require a Net Force to create the change in motion
this acceleration may include a change in direction or speed

10. Mass vs Weight
mass: quantity of matter measured in kg. weight: force (Fg) caused by acceleration due to gravity measured in N. **mass of an object is constant, but weight is dependent upon gravity.

11. Now you try: GIZMO – Weight and Mass
Mass vs Weight
Newton’s 2nd Law says F = m a but we can say
Fg = m ag or F = m g
Calculate the weight of a 3.87 kg box on Earth (ag = -9.8m/s2).
Fg = (3.87)(-9.8) =  F = N or Fg = 37.9 N
Calculate the weight of a 3.87 kg box on the Moon (ag = -1.6m/s2).
Fg = (3.87)(-1.6) =  F = N or Fg = 6.19 N
Now you try: GIZMO – Weight and Mass

12. Newton’s 2nd Law Fnet = m a Acceleration means “change in motion”
all accelerations require a Net Force to create the change in motion
this acceleration may include a change in direction or speed
Fnet = m a

13. Newton’s 2nd Law At equal accelerations: Bigger mass means bigger force.
F= m a F= m a

14. Newton’s 2nd Law At equal masses: Bigger acceleration means bigger force.
F= m a F= ma
Compare easing away from a stop sign slowly increasing speed to 30 mph and peeling out to quickly reach 30 mph.

15. G.U.E.S.S. method
You will use this method to work all math problems in this class. It is a 5-step process to organize and solve math questions in physics.
G: given
U: unknown
E: equation
S: substitution
S: solution

16. Ex1:How much force is needed to accelerate a 68
Ex1:How much force is needed to accelerate a 68.0 kg skier at a rate of 1.20 m/s2? G
m=68.0 kg
a=1.20m/s2 U
F=? E
F=ma S
F=(68)*(1.2) S
F=81.6N

17. F=ma divide both sides by mass a = m F
Ex2: What is the acceleration of a 0.35kg ball that is hit with a force of 25N? G
m=0.35 kg
F=25 N U
a=? E
F=ma S
a=(25)/(0.35) S
a=71m/s2
Algebra is required to isolate the
unknown variable. So …
F=ma divide both sides by mass
to isolate acceleration.
a = m F
use: m m
Acceleration is directly proportional to Force
Acceleration is inversely (opposite) proportional to Mass

18. Newton’s 3rd Law Law of Action/ Reaction
Every action (force) has an equal and opposite reaction (force) .

19. Newton’s Third Law If an object A puts a force on object B,
Then object B exerts an equal force
Back on object A. A B
Amazing Race – Watermelon Smash

20. Newton’s 3rd Law
What are the two forces that will occur in this picture?
Action –
Reaction –
the hammer hits the nail
the nail hits the hammer

21. What force pairs are in this picture?
Newton’s 3rd Law
What force pairs are in this picture?
Action:
the ball pushes on the net
Reaction:
the net pushes on the ball

22. What force pairs are in this picture?
Newton’s 3rd Law
What force pairs are in this picture?
Action:
foot pushes down on the ground
Reaction:
ground pushes up on the foot

23. How does each of Newton’s Laws apply?

24. How does each of Newton’s Laws apply?

25. How does each of Newton’s Laws apply?

26. because of 3rd Law….. Ff is always opposite motion! FN is opposite Fg
FN = 25 N
Fg = 25 N
Ff is always opposite motion!
FN is opposite Fg
Fnet = = 30N
acutal Force effecting the object
is 30N (right)
FA = 40 N
Ff = 10 N
5.0 kg
If the object’s mass is 5.0 kg, what acceleration will it experience?
Vertically Fnet = 0 N …… no vertical acceleration
Horizontally Fnet = 30 N … acceleration ????
Fnet = ma using the Net Force acting horizontally.
to solve for a ……. a = F / m
a = (30) / (5) = 6.0 m/s2
Horizontally Fnet = 30N (right) … 6.0m/s2 (right) acceleration

27. Coefficient of Friction
friction is caused by contact between rubbing surfaces (“rough” tiny bumps – even smooth surfaces have them on the microscopic level).
static friction – initial friction to overcome for a “body at rest”
kinetic friction – friction while an object is “moving”

28. Coefficient of Friction
Ff = μ m g
Force of friction
coefficient of friction
acceleration due to gravity
mass
remember …. “m g” represents the same thing as “m a”
if given mass …. you must multiply by gravity
if given force or “weight” ….. do not multiply by gravity 1. 2.
A 2.1 N ball rolls across the yard. A force of friction of 3.5 N brings the ball to a stop. What is the coefficient of friction between the ball and the yard?
Billy pushes a 28 kg box across the floor. If the floor has a static coefficient of friction of 0.075, what force is needed to move the box?
Ff = μ m g
Ff = (.075)(28)(-9.8)
Ff = 
Ff = -21N
Ff = μ m g ….. Ff = μFg
3.5 = μ (2.1) … ÷ (2.1)
μ = (3.5)/(2.1)  1.66…
μ = 1.7